In communication systems a modem is used to convert (modulate) digital signals generated by a computer into analog signals suitable for transmission over telephone lines. Another modem, located at the receiving end of the transmission, converts (demodulates) the analog signals back into digital form. The transmission speed of digital subscriber loop (DSL) modems has exhibited a remarkable increase in recent years, and as the increase of the transmission speed progresses, it becomes increasingly necessary to reduce the occurrence of errors in data communication arising from disturbances of the circuit to as few as possible.
In addition, due to the large range of different twisted pair loops over which high speed modems operate and the varying amount of interference, the received signal at the analog-to-digital converter input (in the demodulation section of a modem) can present a dynamic range of over 70 dB.
An automatic gain control circuit is provided in the demodulation section of a modem to monitor the input signal level and to provide the appropriate gain to bring the signal to a desired level.
Traditional AGC circuits in modem receivers working over the necessary full dynamic range present significant differences in performance, convergence times, and gain variability at different regions of operation.
Consequently, there is a need for a AGC circuit in an modem receiver that provides gain tracking for a large range of twisted pair loops with over 70 dB range (for example), various signal bandwidths, and in the presence of radio frequency (RF) and other asynchronous digital subscriber line (ADSL) interference. Further, the AGC circuit must improve gain ripple after convergence to minimize its noise contribution to the system.